docs: def nsummary(self,prn=None, lfilter=None):
          """prints a summary of each SndRcv packet pair with the pair's number
  prn:     function to apply to each packet pair instead of lambda s, r: "%s ==> %s" % (s.summary(),r.summary()) 
  lfilter: truth function to apply to each packet pair to decide whether it will be displayed"""
          for i, (s, r) in enumerate(self.res):
              if lfilter is not None:
                  if not lfilter(s, r):
                      continue
              print(conf.color_theme.id(i,fmt="%04i"), end = " ")
              if prn is None:
                  print(self._elt2sum((s, r)))
              else:
                  print(prn(s, r))